Maintained by Robin Tecon, microbiologist and postdoctoral researcher at the Swiss Federal Institute of Technology Zürich. This blog is about bacteria (and other microbes) and the scientists who study them.

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Monday, November 04, 2013

Modelling the bacterial colonization of leaves

Photo courtesy of Jan Tech

Our world
is a quite green world: a sea of trees, bushes, grasses, or, if you happen to
live in the Midwest, corn fields… What is less obvious, though, is the fact
that this green vastness harbors a huge community of microbes. Yeasts and
filamentous fungi are often found on plant surfaces, but the most numerous
inhabitants are first and foremost bacteria. Indeed, a centimeter square of
leaf can contain as many as 10 millions of them! No worries, most of them are
harmless to us or their plant hosts. On the contrary, many are required to
maintain a healthy plant environment, by stimulating plant growth or by
preventing the plant colonization by pathogens (they compete for the same space
and the same resources).

Because
plants are so vital to us (think food, raw materials, landscapes, etc.), there
is a real interest in understanding what the microbial contribution to the
plant ecosystem is. One lingering question, for instance, is how bacteria colonize
the surface of leaves (what we call the phyllosphere). What we do know is that bacteria on leaf surfaces appear as clusters of cells, rather than an even layer of bacteria covering the
surface; the mechanisms that lead to this colonization pattern, however, is not
well understood. I have already written about this question in a previous post that dealt with the use of bacterial bioreporters. Another way to explore these
mechanisms of cluster formation is computer-based modelling, which enables us
to test different scenarios and compare it with what has been observed on real
plants.

Bacteria (in green) colonize the surface of a snap bean leaf and form clusters.

To predict
the clustering of bacteria growing on a surface, Annemieke has created a
so-called individual-based (or agent-based) model1. It differs from
a ‘classical’ model in that it doesn’t try to express the behavior of a whole
population using appropriate mathematical expressions. Instead, a
population pattern emerges from the behavior of individual agents (for instance
bacteria) that obey a few simple rules. In our case, we give the bacteria rules as to how they can grow and form microcolonies. The advantage of
such models is that they do not impose any predetermined behavior on the
virtual population, but rather allow complexity to be derived from simple,
individualistic actions.

Bacteria (in green) clustering next to a leaf stomate.

The main
goal of our model was to explain the clustering patterns of bacteria that were
observed in a classic paper by Jean-Michel Monier and Steve Lindow (2004), from
UC Berkeley. Bacteria can grow on pant leaf surfaces thanks to the availability
of plant sugars such as glucose and fructose. These sugars are located in the
sap, but some of it diffuses from the interior of the plant to the exterior, at least if
water is available on the surface to collect these molecules! For this reason,
the presence of water is a key parameter in our model: if the surface is dry,
bacteria cannot grow! When water is present, sugar is available to the
bacteria; they use it, grow and produce offspring that colonize the surface.
Annemieke thus tested the importance of different ‘waterscapes’ on bacterial
growth, for instance comparing a continuous water film with individual droplets
spread across the leaf surface.

The video below shows a simulation of colonization run with our model. The surface is represented in 2D (right side in the video), and the round elements represent droplets of water. When the droplets turn dark, it indicates that the concentration of sugar has increased. Individual bacteria are seen in green. They will consume the sugars (the color in the droplet becomes white), divide and form microcolonies containing many individuals.

This
mechanism (droplet distribution, sugar diffusion, bacterial growth), however,
was not sufficient to explain the experimental patterns seen by Monier and
Lindow. What was missing was bacterial detachment: when bacteria can detach and
disperse from the microcolony in which they originated (as shown in the video), the model predictions
match the experimental results very accurately! (See figures below.) To say it differently, if
there is no bacterial movement (either active or passive), we cannot explain the
experimental observations, namely that we see some large clusters of thousands
of cells and many smaller clusters of only a couple of cells.

Experimental results by Monier and Lindow.

Predictions of the model without detachment.

Predictions of the model with bacterial detachment.

The main finding
of our study was thus that bacterial detachment from clusters (and
re-attachment) is an important mechanism in the colonization of plant leaves,
and potentially in the colonization of other natural and artificial surfaces. This gives us a better view of what it means to live a microbe's life!

Notes:

1. The
model was built in NetLogo, a platform developed at the Northwestern
University (Illinois) that is freely available!

2 comments:

Very interesting and stimulating. The model describes why epiphytic bacteria population was rather low under our semi.arid conditions of Giza, Egypt field with sunflower as test plant. Hegazi, Cairo Univ. Hegazinabil8@gmail.com

Very interesting and stimulating. The model describes why epiphytic bacteria population was rather low under our semi.arid conditions of Giza, Egypt field with sunflower as test plant. Hegazi, Cairo Univ. Hegazinabil8@gmail.com